1. Field
Apparatuses and methods consistent with exemplary embodiments relate to magnetic resonance imaging (MRI), and, more particularly, to acquiring T2-weighted imaging data using multi-slice imaging.
2. Related Art
When a substance such as human tissue is subjected to a uniform magnetic field, i.e., a static magnetic field B0, the individual magnetic moments of the excited nuclei in the tissue attempt to align with the static magnetic field B0, but precess about it in random order at their characteristic Larmor frequency. If the substance is subjected to a magnetic excitation field B1 that is in the x-y plane and that is near the Larmor frequency, the net magnetization aligned moment Mz may be rotated, i.e., tipped, into the x-y plane to generate a net transverse magnetic moment Mt. An MR signal is emitted by the excited nuclei, i.e., spins, after the excitation magnetic field B1 is terminated, and the MR signal may be received and processed to form an image.
In MRI systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude is determined by the magnitude of the transverse magnetic moment Mt. The amplitude of the emitted MR signal decays exponentially with time.
The T2 time constant is referred to as the spin-spin relaxation constant, or the transverse relaxation constant, and is characterized by a spin-spin relaxation time characterizing the signal decay. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation magnetic field B1 in a perfectly homogeneous magnetic field.
The biological tissues have different T2 values and this property may be exploited to enhance the contrast between the tissues. Accordingly, T2 serves as an informative MRI parameter, providing non-invasive measurements of tissue status and disease prognosis with respect to a wide range of applications and diseases, including imaging of heart, brain, liver, etc.
One technique which uses T2 imaging is a quantative T2 mapping, in which the T2 decay curve is sampled at multiple points, to estimate a T2 value.
In detail, the quantative T2 mapping uses a balanced steady-state free-precession (bSSFP) or gradient echo (GRE) imaging along with T2 magnetization preparation (T2Prep) for pixel-wise T2 mapping. In this technique, multiple single-slice images are acquired with different T2 preparation echo times to obtain multiple images with varying T2 weightings.
However, in the related art T2 imaging, multiple T2-weighted single-slice images are acquired with rest periods of 3-6 seconds inserted in-between the data acquisitions, to allow for full signal recovery before application of a T2Prep with a new T2 value. During the rest period, no magnetization pulses are applied and no image data acquisition is performed. For example, in cardiac imaging, with a three heartbeats rest period used, only three T2-weighted images over 12 heartbeat acquisition are acquired, resulting in data acquisition efficiency of 25%. Therefore, to cover the entire left ventricle (LV), for example, with five slices, the scan time is 60 sec of which 45 sec are a waiting time with no data acquisition, leading to long scan times.
Furthermore, in some cases, when more T2Prep samples are needed for more precise and reproducible T2 maps, the related art scans become longer.
Also, recently, the three-dimensional (3D) T2 mapping sequences have been proposed, which require even longer acquisition time, e.g., 18 min, to cover the entire left ventricular (LV), in cardiac imaging.
Accordingly, apparatuses and methods are needed to provide accurate quantitative T2 mapping in a short amount of time, with reliable reproducible measurements.